Installations of the above-described type are well known and are usually referred to as hydroelectric stations with pumped energy storage. During periods of peak electricity consumption, the function of stations of this kind is to provide electrical power to the electricity distribution supply network with which they are interconnected. During periods of slack electricity consumption, the excess capacity of the distribution supply network is used to refill the upper reservoir by pumping. Energy is thus stored by accumulating (raising) water in the upper reservoir. During peak periods, the hydroelectric station supplies electricity to the distribution supply network by running the water stored in the upper reservoir down to the lower reservoir through a turbine.
In another connection, large built-up areas, because of the area of land they cover and because of the magnitude of their populations, make it particularly difficult to treat various problems relating firstly to nature and secondly to human activity. Amongst these problems, particular mention may be made of:
1) spreading out peak flows in the sewage and rainwater networks (drainage network); and PA1 2) providing electrical power during periods when demand is particularly high, particularly in the morning and in the evening in winter time, but possibly also during slack periods in the event of a failure in an electricity production unit or in an electric power transmission line. PA1 a) discharge excess polluted water directly into the natural environment, thereby giving rise to consequences that are becoming less and less acceptable to the population; PA1 b) undertake massive investments to match treatment capacity to peak flow rates, which strategy is difficult to justify on economic grounds; and PA1 c) increase the retention capacity of the main drainage network so as to spread out the peak flow and stagger the treatment of the excess volume of water. Such an increase in capacity may be obtained either by increasing the size of the main sewers and other main drains of the drainage network, or else by adding storage reservoirs connected to said network. Unfortunately, there is generally no room to locate large reservoirs within or close to built-up areas, and in addition the price of land is generally very high in such areas. PA1 a) hydroelectric stations situated on water courses; and PA1 b) thermal power stations fitted with generators that can be started quickly, such as gas turbines.
With respect to spreading out peak rainwater flows, in an urban environment, rainwater mixed with sewage water must be subjected to physico-chemical and bacteriological treatments prior to being discharged into the natural environment, because of the numerous polluting factors carried by the water. In the present state of the art and in existing installations, sewage works or other plants for performing this treatment can absorb medium flow rates only. When confronted with occasional peak flow rates that exceed the treatment capacity of such works and sometimes also the capacity of the sewers, planning authorities can adopt various strategies:
Under such conditions, it will be understood how advantageous it would be if large rainwater storage reservoirs could be created in the immediate proximity of large built-up areas, or even within them.
Returning to the supply of electricity during peak periods or in the event of a failure in an electricity production unit, matching electricity production to demand peaks requires installations capable of varying their rate of production very quickly from zero to a maximum value. There are essentially two types of installation capable of satisfying this requirement:
Unfortunately, such stations must be located at a certain distance form built-up areas. The mere fact that they produce electricity requires new electric power transmission lines to be installed and this is becoming more and more difficult in urban and near-urban fabric.
Under such conditions, it will be understood how advantageous it would be if electricity production systems could be located in the immediate proximity of large built-up areas, or even within them.